1. Field of the Invention
This invention pertains to friction stir welding and more particularly to a friction stir weld tool with an improved probe structure for welding high-strength aluminum alloys and other hard materials such as steel, copper, nickel, titanium and their alloys.
2. Background and Objects of the Invention
Friction stir welding is a relatively new welding technique discovered in the mid, 1990s that was developed primarily for welding aluminum and soft aluminum alloys that were difficult to weld using traditional welding techniques. The technique uses a rotating shouldered cylindrical tool with a projecting pin to generate heat in the workpiece. The mechanical friction of the rotating tool contacts the workpiece and plasticizes (softens) the metal as it is plunged into the bondline. At this stage, there is a substantial amount of plasticized metal in a column about the rotating pin beneath the shoulder of the cylinder portion of the tool. The tool is then moved along the bondline relative to the workpiece. As the pin rotates and moves in a transverse direction, the metal is plasticized at the front of the pin and extruded to the back of the pin while undergoing a mechanical stirring and forging action imparted by the pin surface profile and confined from above by the pressure exerted on the material by the shoulder of the cylindrical tool. The plasticized metal is transferred from the front of the pin around the periphery of both sides of the pin and subsequently reconstituted at the back of the pin to produce the weld. The rotational and translational (travel) speed are controlled to maintain a plasticized metal state.
The stir weld tool is formed as a cylindrical piece with a shoulder face that meets a pin (probe) that projects from the shoulder face at a right angle (U.S. Pat. Nos. 5,460,317 and 6,029,879). In some instances, the probe actually moves in a perpendicular direction in an aperture formed in the face of the shoulder (U.S. Pat. No. 5,697,544; U.S. Pat. No. 5,718,366; and U.S. Pat. No. 5,893,507). The face of the shoulder can be formed with an upward dome that is perpendicular to the pin (U.S. Pat. No. 5,611,479; U.S. Pat. No. 5,697,544; and U.S. Pat. No. 6,053,391). The dome region and an unobstructed dome (shoulder face)/pin interface are considered essential for the proper frictional heating of the workpiece material. The dome region constrains plasticized material for consolidation at the trailing edge of the friction stir tool rather than permitting it to extrude out from under the sides of the tool. A clean, unobstructed shoulder face/pin interface are considered critical for proper mechanical flow, stirring and mixing of the plasticized workpiece material around the pin during welding and the subsequent mechanical and metallurgical properties of the resultant joint.
Another factor influencing friction stir weld design is the translational (travel) speed at which the tool moves along the bond line. Although tool speeds of 7-15 inches/minute (180-380 mm/min are common in xc2xc inch (6 mm) aluminum-alloy stock (BS6082; U.S. Pat. No. 5,460,317), the pin of the tool has a tendency to break as the travel speed or the thickness and/or hardness of the material to be joined increases. Although typically the friction stir weld material is a tool steel material such as H13 (U.S. Pat. No. 6,053,391), increasing the hot shear strength of the tool material alone or the diameter of the pin has not been the answer to welding thick sections of material, i.e., greater than about 1 inch (25 mm) in thickness (xe2x80x9cDevelopment of Improved Tool Designs for FSW [Friction Stir Weld] Aluminumxe2x80x9d, Chris Dawes and Wayne Thomas, The Welding Institute (TWI), Friction Stir-Welding Symposiumxe2x80x9d June 1999, Thousand Oaks, Calif.).
Several attempts have been made to avoid the pin breakage problem. When welding thick or hard-material sections, travel speed can be reduced to avoid tool breakage. However, this brings about its own problems. At such slow speeds, the workpiece metal overheats and a dramatic loss in joint efficiency, often of the order of 10-15%, occurs. In addition, the welding process may become so slow as to be economically infeasible. Another solution is to use a double pass welding technique in which the pin is only plunged into the workpiece to slightly over half its thickness and then welded again from the opposite side of the workpiece. Although tool speed and breakage problems are decreased, material handling time and cost increase substantially and the technique may not be applicable where the workpiece configuration prevents access to the second side. Finally considerable effort has been put into the design of the tool pin (probe) with a variety of pin designs having been made (U.S. Pat. Nos. 6,053,391; 6,029,879; 5,460,317 and xe2x80x9cDevelopment of Improved Tool Designs for FSW [Friction Stir Weld] Aluminumxe2x80x9d, Chris Dawes and Wayne Thomas, The Welding Institute (TWI), Friction-Stir Welding Symposiumxe2x80x9d June 1999, Thousand Oaks, Calif.). However, none of the designs satisfy the need for friction stir welding tool pins that allow for high-speed welding in thick, strong, or hard materials without breakage. In fact, the introduction of sharp angled threads and bosses into the tool may well be one of the sources of high-stress concentration that leads to pin failure.
As such, friction stir welding has been limited to relatively soft materials such as plastics and soft aluminum alloys in the 1xxx, 5xxx, and 6xxx aluminum series for joints with thickness in the ranges of {fraction (1/32)} in (0.79 mm) to xc2xd in (13 mm). For stronger aluminum alloys such as the 2xxx and 7xxx series, workpieces of a thickness less than about xc2xd in (13 mm) are the limit for good stir friction welds.
Unfortunately, as the joint thickness is increased above about xc2xd in (13 mm) thick on aluminum alloys with ultimate tensile strengths greater than about 50 ksi at welding speeds above 2 inches/min, friction stir welding becomes a less effective joining process due to tool failure or short tool life. Typically, most tools built according to the teachings of the state of the art, fail after only a few inches of travel when welding in harder materials. Typically the length of travel before pin fracture is less than 12-inches of travel leaving the fractured pin solidified in the workpiece.
Accordingly, it is an object of the present invention to provide a friction stir welding tool. capable of welding high strength materials while providing high strength in the resulting weld without pin fracture.
It is another object of the present invention to provide a friction stir welding tool capable of welding thick materials while providing high strength in the resulting weld without pin fracture.
It is another object of the present invention to provide a friction stir welding tool capable of welding thick high-strength materials while providing high strength in the resulting weld without pin fracture.
It is another object of the present invention to provide a friction stir welding tool capable of welding high-strength and/or thick materials at relatively high speed.
It is another object of the present invention to provide a friction stir welding tool with a thread pattern that reduces areas of high-stress concentration.
It is another object of the present invention to provide a friction stir welding tool that avoids the need for double pass welding.
It is another object of the present invention to provide a friction stir welding tool that increases the speed of the welding process.
It is another object of the present invention to provide a friction stir welding tool that decreases the cost of the friction .stir welding process.
It is yet another object of the present invention to provide a friction stir welding tool that results in high joint efficiency.
In order to meet these objects, the present invention of a friction stir weld tool comprises a cylinder having a first end for attachment to a rotating drive, a longitudinal axis, a shoulder, and a shoulder face that is substantially perpendicular (normal) to the longitudinal axis. An integral probe (tip) projects from the shoulder face with an axis that is co-extensive with the cylinder axis. The invention features an integral transition geometry structure between the shoulder face and the probe that has the advantage of substantially reducing rotary bending fatigue and subsequent breakage of the tool. Typically the transition geometry structure is a curved transition geometry structure that typically has a radius of curvature of at least about 0.040 inches+0.015(txe2x88x920.010)1.5 where t is the length of the probe from its base to its distal end.
Another feature of the invention is the selection of a tool material on the basis of mechanical properties such as the ultimate tensile strength at the processing temperature of the work piece material rather than at room temperature. For example, it has been found that the ultimate tensile strength of the tool decreases significantly in going from room temperature to the tool operating temperature. Selecting the tool material on the basis of its ultimate tensile strength at the tool operating temperature rather than at room temperature has the advantage of dramatically reducing the amount of tool breakage.
Another feature of the present invention is the introduction of compressive stress in areas of fatigue cracking such as at the base of the probe and in the threaded area of the probe. Such stress may be introduced by mechanical processing such as peening and shot blasting or by chemical processing such as nitriding, carbonizing, and carbonitriding and have the advantage of improving fatigue resistance.
Yet another feature of the instant probe is the modification of the probe threads by adding a generous root radius (rounded root) at the root of the thread and using a ratio of the major diameter of the threads to the. minor diameter of the threads that increases from the base of the probe to its distal end. This has the advantage of improving dramatically the life of the tool by substantially reducing the high stress concentrations inherent in sharp threads and especially those found at the base of the probe where the stresses are at their highest.
By using one or more of these features, a friction stir weld tool can be made that is capable of welding a wide variety of thick, high strength materials including aluminum, aluminum-based alloys, copper, copper-based alloys, iron, iron-based alloys, titanium, titanium-based alloys, nickel, nickel-based alloys and combinations thereof.